Much effort has been expended to limit bit error rates in long haul optical communication systems, i.e. systems having bit rates greater than 10 Gbps and a distance between reshaping points for the optical pulses of more than 100 Km. For a given transmission distance, a variety of dispersion effects significantly influence this bit error rate. For example, chromatic dispersion involves effects stemming from the dependence of fiber traversal rate on signal wavelength. Since the signal is always a spectral composite of different wavelengths (although often a very narrow spectral composite) one end of such spectrum trails the other as the fiber is traversed. This spreading of the signal leads to the overlap of adjacent signal pulses and the concomitant increase in error.
Initially to mitigate this error optical fiber was employed in long haul systems that had a wavelength (approximately 1300 nanometers) where dispersion (but not the change of traversal rate with wavelength) was essentially zero. Thus wavelengths closely surrounding 1300 nm experience little dispersion. However, as fiber run distances became longer, systems were designed to operate in the region between 1480 and 1610 nanometers rather than around 1300 nm because, among other things, the theoretical minimum signal attenuation per kilometer for glass fiber is at approximately 1550 nanometers. In addition, wavelength division multiplexed signals are most efficiently amplified together rather than by separating the channels and individually providing amplification. Multichannel amplifiers such as those used in long haul systems have a useful operating range around 1550 not 1300 nanometers.
For typical operating parameters used in long haul wavelength-division-multiplex communication, it is essential that the transmission fiber have a non-zero dispersion at the operating wavelength to avoid the significant adverse consequences associated with non-linear effects. As a result, long haul fiber is designed to have non-zero dispersion at 1550 nanometers. The adverse effects resulting from non-zero dispersion have been substantially reduced in such systems using cumulative dispersion compensation modules. These modules contain a suitable length of fiber that produces a dispersion approximately equal to that incurred on the transmission line but of opposite sign. Accordingly, the cumulative transmission line dispersion is substantially compensated. Thus, the use of dispersion compensating modules in long haul applications has become routine.
In summary, long haul communication systems have evolved to approaches employing operating wavelengths in the range 1480 to 1610 nm (designated S, C, and L bands) with typical standard single mode fibers (SSMF) having significant dispersion at such operating wavelengths and with the effects of such dispersion compensated using a dispersion compensating module. Because system penalties due to dispersion are compounded by large transmitter spectral widths, single longitudinal mode lasers (SLMs) that have narrow spectral widths are routinely favored over multi-longitudinal mode lasers (MLMs). Such SLMs are quite expensive (presently as much as several thousand dollars depending on bit rate and other requirements). However, since SLMs emit a single longitudinal mode, complications involving wavelength broadening due to multiple laser modes are avoided.
The approach is significantly different for short haul transmission, i.e. transmission lengths between transmitter and receiver of less than or equal to 20 Km. In local area networks presently contemplated for high bit transfer rates (greater than 1 Gbps) between, for example, a local central office and a consumer, the use of multiple SLMs is not economic. Typically, for such systems, MLMs having a current price on the order of $50 (fifty dollars) appear to be an economic necessity. Such lasers generally emit in the wavelength range 1260 to 1360 nm and thus transmission fiber (e.g. SSMF) having zero dispersion near 1310 nanometers is usually employed with such lasers. Since local area networks have fiber lengths between sender and receiver of 20 Km or less the lower attenuation of the 1500 nanometer operating range is not essential, and the use of fiber having a zero dispersion at approximately the operating wavelength is employed beneficially. This benefit seems especially advantageous since non-linear effects are substantially less significant for such local area networks and dispersion compensation at 1300 nm does not seem needed.
Nevertheless, the use of MLMs introduces an error source not present when SLMs are used. In particular, an effect denominated mode partition noise (MPN) becomes significant. (See Agrawal, G. P. et. al. Journal of Lightwave Technology, 6(5), 620 (1988) and Ogawe, “Analysis of Mode-Partition Noise in Laser Systems,” IEEE Journal of Quantum Electronics, QE-18, 849 (1982) which are hereby incorporated by reference in their entirety for a discussion of MPN.) This noise results from the interaction of the fiber dispersion with the constantly changing power distribution among laser modes. A typical instantaneous power distribution among modes for MLMs is shown in FIG. 1. As time progresses, the power present in each such mode changes due to factors such as fluctuations in drive current and diode temperature. The manner of such change is typically quantified by a mode partition coefficient, k, that is given values between zero and one. A laser having k=1 emits all its power in a single mode but that mode having the entire power output is constantly changing. In such a laser at time zero the mode power spectrum would be shown in FIG. 2 while at time t=t1, the power output would be in a different mode at a different wavelength, as shown in FIG. 3. For a laser with mode partition coefficient equal 0, all modes are emitted at the same time and the distribution of power remains substantially constant with time.
A laser having a mode partition coefficient equal 1 is the least desirable since the entire power of the laser continuously shifts all from one wavelength to all in another wavelength. Although lasers with low partition coefficients are more desirable, they are difficult to reliably produce, and thus it is not currently feasible to base a system on a requirement for low partition coefficient lasers.
However, even for lasers of relatively low mode partition coefficient, (k, in the range 0.2 to 0.5) the transmission of a multitude of modes (typically between 30 and 50 detectable modes) having a spectrum of wavelengths (modes) exhibiting time dependent power fluctuations yields a significant source of error. As denominated by Agrawal et.al. supra, this error results from the interaction of these transmitted modes having time dependent power fluctuations and the wavelength dependent effects introduced by the fiber. Further, the mode spectrum of the laser varies with temperature often changing as much as 0.3 nanometers per degree centigrade. This temperature effect also contributes to the difficulty associated with the use of MLMs because the center wavelength shifts and results in a change in wavelength dependent fiber dispersion. In addition to these temperature-dependent spectral effects, there is (for a given temperature) significant variation in center wavelength (up to 100 nanometers) among lasers.
Generally, the method employed to satisfactorily reduce MPN has been to narrow the spectrum actually injected into the fiber by employing expedients such as fiber Bragg gratings, high-pass filters, or even to revert to the more expensive SLMs. (See, for example, Journal of Lightwave Technology, 21(9), 2002 (2003) and IEEE Journal of Selected Topics in Quantum Electronics, 7(2), 328 (2001).) Nevertheless an approach that significantly reduces MPN without the substantial increased cost of single mode lasers or the additional cost associated with other optical components is quite desirable.